0 Helpful?

Vapor retarders for double stud cellulose-filled wall assemblies

Following up on last month's blog about building codes, am I correct that the code allows interior vapor retarding with latex paint on sheet rock? Also, does the vapor permeability of ZIP sheathing prevent inner wall condensation without the use of exterior foam?

Asked by Michael Roland
Posted Aug 7, 2014 12:53 PM ET
Edited Aug 7, 2014 2:38 PM ET


4 Answers

Select your preferred way to display the comments and click "Save settings" to activate your changes.

What works and what is allowed by code is country/climate-zone dependent. Where are you?

Answered by Dana Dorsett
Posted Aug 7, 2014 2:17 PM ET


For information on vapor retarder requirements in the IRC and IECC, see this article: Vapor Retarders and Vapor Barriers.

Since 2007, the IECC has required (in section 402.5) that walls [with some exceptions] in climate zones 5 (e.g., Nevada, Ohio, Massachusetts), 6 (e.g., Vermont, Montana), 7 (e.g., northern Minnesota), 8 (e.g., northern Alaska), and marine zone 4 (Western Washington and Oregon) have a Class I or Class II vapor retarder — in other words, kraft facing or polyethylene.

The most important exception is for walls with an adequate thickness of continuous insulation on the exterior side of the wall sheathing.

Answered by Martin Holladay
Posted Aug 7, 2014 2:37 PM ET


Dana and Martin,
Thanks for your responses. I believe I understand vapor diffusion through walls, but since some of the GBA details for cellulose-filled wall cavities don't show an interior vapor retarder (and some do), I was hoping to avoid that step. Seems like it's up to the local adopted code and inspector and/or my ability to persuade.
Dana, I'm in upstate NY (Albany area), climate zone 5.

Answered by Michael Roland
Posted Aug 8, 2014 10:31 AM ET


In climate zone 5 you can use standard latex paint as the interior side vapor retarder on a cellulose-filled wall if you have a vented "rainscreen" gap between the plywood or OSB or fiberboard sheathing, per IRC 2012 chapter 7:


This works with standard vinyl siding too (since it's inherently back ventilated) but not the foam-filled vinyl siding (unless you build in a rainscreen.)

Cellulose is fairly protective of plank sheathing even when the siding is tight to the housewrap due it's ability to take on substantial moisture without damage or loss of function, but if the sheathing is OSB it's not worth the risk to skip it. The risk to plywood is somewhat in-between.

Answered by Dana Dorsett
Posted Aug 8, 2014 10:50 AM ET

Other Questions in Building Code Questions

Making sense of 1st month mini split heating data

In Mechanicals | Asked by kjmass1 | Dec 12, 17

Exterior Mineral Wool Insulation: Batts vs denser boards

In General questions | Asked by matthias paustian | Dec 12, 17

1952 Cape Cod

In Plans Review | Asked by Emerson W | Dec 11, 17

Workshop w/ Dwelling Above - Heating Advice

In General questions | Asked by user-6964558 | Dec 12, 17

Brick house insulation

In GBA Pro help | Asked by 3rd Little Pig | Dec 9, 17
Register for a free account and join the conversation

Get a free account and join the conversation!
Become a GBA PRO!